This invention relates to a method for electroplating, and more particularly to a method for electroplating a metallic layer onto a surface of a photovoltaic device absent any external electrical contact to the surface.
Photovoltaic devices, and more particularly solar cells, often consist of a semiconductor wafer which forms a single large PN junction. Electromagnetic radiation such as sunlight incident upon that junction produces electrical carriers in the device and generates an electrical current. To be useful, that current must be collected and conveyed to an external circuit. The collecting is accomplished by metallic patterns which are in ohmic contact with the two sides of the PN junction. Because the photovoltaic device generates high currents at a very low voltage (essentially the forward voltage of the PN junction) it is necessary that the metallic pattern provide a low resistance path to minimize resistive losses of the generated current. The metallic pattern must be limited in its physical extent, however, especially on the front surface of the device, in order to minimize the amount of incident radiant energy intercepted by the metal and thus lost for current generation purposes. In sum, this requires that the front metallic pattern consist of narrow strips of very highly conductive material. The requirement is exacerbated in so-called concentrator cells upon which the incident radiation is concentrated and can be many times higher than the normal incident radiation. Because the generated current is roughly proportional to the incident radiation, this can result in very high current densities and the need for a very low resistance metallic pattern.
In fabricating a flat plate silicon solar cell designed for one sun operation, a satisfactory metal pattern can be achieved by applying a thin patterned layer of an ohmic contact and barrier layer material and by subsequent solder dipping. The solder dipping provides a thick conductor of low enough resistance for many applications. The solder itself, however, is not of low enough resistance to be satisfactory for concentrator cells. A more highly conductive material such as silver or copper is needed. Heretofore, however, there have not been any satisfactory, economical, efficient processes for providing thick layers of the desired high conductivity materials. Vacuum processes such as sputtering or filament evaporation are too time consuming and expensive. Electroless plating is unsatisfactory because there is no satisfactory electroless bath for the plating of either silver or copper. In addition, all of the conventional electrolytic techniques have severe drawbacks. For example, to plate a thick layer of silver onto the thin ohmic contact and barrier layer material, electrical contact must be made to that material which then functions as one of the electrodes of the plating reaction. If contact is made to the material at a single location or at a finite number of discrete locations, however, there will be a large resistance drop from that contact or contacts to the distant portions of the metal pattern. This high resistance results from the normally high resistivity of the thin layer of material. As the plating starts, this resistance causes distant portions of the metal pattern to be at a lower potential than those portions of the pattern adjacent contacts. This causes the plated layer to be very non-uniform in thickness since the rate of plating is proportional to the applied voltage which establishes a plating current density associated with a particular applied voltage. One possible solution to this problem is to plate very slowly at very low plating currents. Because the current is low, the ohmic drops are low and potential variations across the wafer can at least be minimized. This solution is not economically acceptable, however, because the plating is too slow. The problems encountered in electrolytic plating become even more pronounced when one tries to simultaneously plate onto both sides of the semiconductor wafer, whether the back metallization is patterned or not.
Accordingly, it is an object of this invention to provide an improved electroplating method which overcomes the limitations inherent in prior art methods.
It is a further object of this invention to provide a method for the uniform plating of metallic layers onto both major surfaces of a device.